The invention relates to electrical transformers in general and more particularly to a hexagon transformer as can be used for AC to DC power conversion in a 12 pulse transformer/rectifier arrangement.
The invention is particularly applicable to transformer/rectifier apparatus such as described in U.S. Pat. No. 4,255,784 of John Rosa. As explained in the patent, a hexagon transformer/rectifier scheme has been implemented with a single three-phase transformer having two identical secondary windings per phase between which the associated primary winding can be sandwiched. Such an arrangement allows a symmetrical construction resulting in well matched voltages and leakage reactances and leading to a significant reduction in the input a.c. harmonic distortion levels and d.c. output ripple level. The major problem in the implementation of a hexagon transformer such as suggested in the Rosa patent is due to the fact that a thee-leg magnetic core lies in a plane about a center leg, whereas the hexagon is vectorially distributed evenly in space. Therefore, in the conventional magnetic core, the leakage inductance between the primary and secondary windings which are wound on the center leg of the magnetic core is different from the leakage inductances relative to the outer legs. In addition, the leakage inductances have to be equal for the two ends of each of the six secondary windings in order to avoid an asymmetry. Beside the symmetry problem, with a three leg magnetic core the high leakage inductance causes relatively large stray magnetic fields in the vicinity of the two outer transformer legs. As a result, metallic objects in close proximity will affect the leakage inductances. Also, undesirable losses and heat problems may result due to the presence of nearby cabinet walls or coupling structures.